The present invention relates to telephone systems and more particularly to improved range extender circuits for long subscriber loops.
The portion of telephone circuit between the central office and the customer's station set usually consists of a twisted pair of wires. A maximum acceptable circuit resistance between the central office and the customer's equipment is often imposed since the longer the loop, the greater the attenuation and distortion of telephone signals. Signaling, i.e., transfer of nonvoice information such as dial pulsing, ringing and tripping of the line relay when a call is answered, may be impared by long subscriber loops. The latter may for example either reduce the loop current to the point where central office switches become inoperative, or distort trains of dial pulses resulting in erroneous connections. Similarly, long loops affect the transmission of voice signals by reducing the voice level or, by impairing the operation of the telephone transmitter.
Various known arrangements have been proposed to overcome transmission and signaling difficulties. One such known arrangement, called REG (Range Extender with Gain), is described in U.S. Pat. No. 3,671,676, issued to J. L. Henry et al. on June 20, 1972, and assigned to the assignee of the present application. In the signaling mode of this known REG, a resistive shunt is applied across the loop with each dial pulse to aid operation of the pulsing relay for an originating call. In case of a terminating call, the shunt is applied upon answer either during ringing or during the silent interval, to aid operation of the ring-trip relay. In the transmission mode, voice-frequency gain and greater transmitter voltage are applied to the loop. A loop current detector senses loop currents to control logic, timing and relay driving circuits thereby enabling shunt action and mode selection. While this known loop current detector operates effectively for its intended purpose, it is not insensitive to common-mode currents induced by ac power lines. Such currents can be exceptionally large on long subscriber loops. These common-mode currents, shunted to ground through resistors, relay or transformer windings, give rise to common-mode voltages. In turn, these voltages can adversely affect electronic circuitry connected across the tip and ring conductors of a subscriber loop. Furthermore, the type of control logic, timing and relay driving circuits included in known REGs does not minimize the cost and physical size of the REG.
Another known subscriber loop range extender is described in U.S. Pat. No. 3,784,756, issued to J. M. Nemchik on Jan. 8, 1974, and assigned to applicant's assignee. This known range extender has a through transmission path, a voice transmission path including a voice-frequency repeater, relay control circuitry for transferring back and forth between paths, and a dual mode loop current detector. In the idle state, the current detector is in a slow mode for the purpose of preventing spurious operation of the detector in response to 20Hz ringing current, except in response to an off-hook current flow. Immediately after detecting an off-hook condition, the current detector is switched to a fast mode for detecting 10Hz dial pulses. In this known arrangement, problems similar to the ones discussed in connection with the above-cited reference to Henry et al. are encountered.
In U.S. Pat. No. 3,819,866, issued to G. T. Hawley on June 25, 1974, and assigned to applicant's assignee, a loop current detector is described wherein loop currents cause one photoresponsive arm of a bridge to be illuminated, while longitudinal currents cause both photoresponsive arms of the bridge to be illuminated. The former provide a bridge output, while the latter leave the bridge in balance. Protection devices are required to protect the photoresponsive elements from high voltages and currents resulting from large longitudinal currents. This known light-coupled loop current detector does not ensure that large common-mode tip and ring voltages will not drive the detector devices out of their active regions of operation.
In some two-party loops, a resistor is inserted in the tip party station set for drawing current to ground when the tip party is off-hook. In such arrangements, the ring party station set does not include a resistor connected to ground. During an Automatic Number Identification (ANI) test, the tip and ring leads are connected together to a battery for determining the status of the station sets. If current is drawn from the battery to ground, it follows that the tip party station set is off-hook. If no current is drawn on the line, this indicates that the ring party station set is off-hook. The control logic circuitry associated with such known ANI test circuits may be set by a momentary break in current coming from the central office. Furthermore, due to the presence of differentiation circuits in the control logic, these ANI test circuits respond to noise signals and are subject to a slow recovery resulting from the time constant of the differentiation circuits.